Magnetic information recording and/or reading head with increased wear-resistant properties and a method for manufacturing the same

ABSTRACT

A magnetic head for information recording and/or reading which comprises a pair of half poles (10,11) each consisting of a plurality of soft iron sheets (12) having predetermined profiles. The half poles are attached to each other to define an air gap (13) in front of a magnetic carrier (16). A coil (15) is arranged on the core. Thin hard coatings (18) are deposited on the surfaces of the sheets (12) at least in the region of the air gap and the frontal contact zone (17) with the carrier, whereby a laminated sandwich structure is obtained, in which the hardness is periodically changing along the width of the carrier. Owing to the rigidity of the carrier, the hard coatings resist to wear caused by the sliding movement of the carrier. The presence of the thin coatings does not decrease the resulting permeability of the core. The coating is deposited by means of high rate reactive cathode sputtering technique, which provides for optimum hardness and cohesion to the iron substrate. A preferable coating material is titanium nitride.

This application is a continuation of application Ser. No. 770,863,filed 7/11/85 now abandoned.

The invention relates to magnetic information recording and playbacktechnique, more particularly to a method for producing a magnetic headusable for information recording playback. The term "informationrecording" covers sound recording on a magnetic carrier, video recordingtechnique, as well as digital information recording. For the sake ofsimplicity the examples disclosed in the present specification willrelate to conventional tape recording applications, however, any otherrecording and reading application is considered as equivalent.

The development of information recording technique has created a smallnumber of head types.

A conventional and widely used head comprises a magnetic core made ofprofiles soft-iron sheets in a laminated arrangement, and there isprovided an air gap between the sheets just across the frontal zonewhich abuts the magnetic carrier. A coil is wound around the core forconnection with appropriate electronic circuits. Such heads are held ina head support and have a magnetic shielding around them. These headsproved to be popular, they had fairly good electrical properties (atleast in sound recording applications) and their manufacture was not tooexpensive. A basic drawback of such heads was that the soft corematerial was soon worn by the frictional effects of the tape.

The development of the art has introduced chromium-dioxide and metaltape materials which have been much harder than normal tapes, and thelaminated cores were unable to endure the wear caused by such medias.

In an improved technology a hard coating is provided on such heads,whereby the surface hardness has become sufficiently high. The problemwith such heads lies in that the coating material increases theeffective air gap to twice its thickness, and in most applications thehigh frequency response of the recording has become much worse thanwithout such coatings.

Another head strucure family is based on the use of ferrite or glassceramic materials. These materials have sufficient hardness to endureincreased wear and they are also preferable regarding their frequencyresponse. A drawback with such heads lies in the comparatively low valueof permeability, whereby their electric signals are at a lower levelthan in case of permalloy cores. The greatest drawback of such headslies in the difficulties during manufacture. These hard materials arehard to be formed and tooled, and their production is costly and itrequires much time and work. A further drawback of such heads lies intheir low heat conductivity. In operation, the effect of friction mightcause extreme temperatures in the vicinity of the air gap, and at theirelevated temperatures a recrystallisation might take place at theboundary surface of the glass and the ferrite material, which virtuallyincreases the air gap and decreases frequency response. The thermalstresses may often lead to small cracks which mean the end of theiruseful life.

The object of the invention is to provide a method for producing amagnetic head which not only has good electrical properties, but can bemanufactured with reasonable costs and has improved wear-resistantproperties.

The invention is based on the discovery that for increasing the hardnessof the soft-cored heads, the hard material should be arranged betweenthe soft-iron sheets rather than on the head. If a hard material likee.g. titanium nitride is deposited on the main surface of the ironsheets forming the core, then the hardness of the so-obtained laminarsandwich structure will vary along the width of the information carrier(the tape) as a comb-like function. Since the tape has sufficientrigidity in the stretched state over small distances, the tape issliding on the hard edges only and causing no wear to the soft ironmaterial between the hard coatings.

This effect is similar to the passing of the wheels of a car above thegrating of a sewer or the like. The wheel cannot get in between the ironrails if the rails are arranged in a sufficient density.

It has been experienced that even as thin coatings as a half micron orless could provide an increased resulting hardness. If the thickness ofthe coating is above about 2 microns, the resulting hardness will notincrease significantly with increasing coating thickness. Thearrangement of the hard material between the sheets results in a furtheradvantage, i.e. the structure remains unchanged after some wear. Whilethe front-deposited hard layer was destroyed following a wear of about 1micron, the sandwich structure according to the invention preserves itshardness throughout the depth of the air gap.

It is preferable if the hard coating is made by deposition, especiallyby high rate reactive cathode sputtering. This manufacture can providefor controlled physical properties by means of appropriate processcontrol during deposition.

The Vickers hardness of the surface can be as high as HV₁₀ =3000 kp/mm²which is twenty times as high as that of the soft iron material.

The resulting wear properties of the head according to the invention arejust as good as those of ferrite and ceramic heads, while the higherresulting permeability of the core material results in higher signallevels, thus better signal to noise ratios. The deposition step does notadd significant costs to the well-known manufacturing technology of softiron cored heads, which means that these heads can be manufactured withreasonable costs.

Several other properties and advantages of the present invention will bedescribed in connection with preferable embodiments thereof, in whichreference will be made to the accompanying drawings. In the drawings:

FIG. 1 is an enlarged elevation view of a half core,

FIG. 2 is a side view of FIG. 1,

FIG. 3 is a schematic arrangement of a head and a tape during operationon an enlarged scale for visualisation,

FIG. 4 is an enlarged sectional view taken along line IV--IV of FIG. 3,

FIG. 5 is a hardness versus width curve for the structure of FIG. 4,

FIG. 6 is an enlarged top view of the frontal zone of FIG. 3 viewedacross the tape,

FIG. 7 is a schematic perspective view of a multichannel head assembly,and

FIG. 8 is a hardness curve similar to FIG. 5 for the head of FIG. 7.

FIGS. 1, 2 and 3 show schematically the magnetic poles of arecording/reading head made according to the invention. The headcomprises a pair of half-poles 10, 11 each comprising a plurality ofprofile sheets 12 of a soft iron material with high magneticpermeabiliy. The sheets 12 are stacked and fixed together by means of anadhesive bonding. The two half-poles 10, 11 are attached to each otheras shown in FIG. 3 and an air gap 13 is defined between them byinserting a non-magnetic foil between their lateral side surfaces(surface 14 in FIG. 1). The width of the air gap 13 is in the micronrange and typically falls between about 0.6 and 10 microns. The heightof the lateral surface 14 defines the full depth of the air gap 13. Acoil 15 is arranged on the half poles which serves as a pick up coil inplayback mode and as a magnetizing coil in recording mode.

FIG. 3 shows the head in operation when tape 16 is pressed aginstfrontal zone 17 and the tape is moving with a predetermined speed in thedirection of arrow A. The frontal zone 17 of the head is made preferablyby a grinding operation and its profile is tooled to provide an optimumguidance for the tape 16.

The head shown in FIGS. 1 to 3 resembles conventional heads withlaminated core, e.g. such as described in the book of Dipl. Ing.Christian Scholz "Magnetbandspeichertechnik" (VEB Verlag Technik,Berlin, 1969, pp. 211-236) or used widely in commercial tape recorders.A basic difference between the head according to the invention and theconventional ones lies in that each sheet 12 comprises a coating 18 onits surface at least in the region defined by the depth of the air gap13 and the frontal zone 17. The coating 18 is made of a hard materialwhich has an increased resistivity against abrasion.

The coating can be made by means of conventional vapour depositiontechnique such as ion plating or cathode sputtering. Since magneticheads are manufactured by massproduction, it is preferable if thecoating 18 is made by means of high-rate cathode sputtering techniquewhich offers not only a high productivity but also a uniform coatingthickness, a perfect adhesion to the substrate material and controllablecoating properties. The high-rate cathode sputtering technique iswell-known in the art, and it is widely used for various applicationsincluding the hardening of cutting edges, coating watch-cases andbracelets and thin-film technique. The high-rate cathode sputtering isdescribed in detail e.g. in the paper of W. D. Munz and G. Hessbergerentitled "Production of hard titanium nitride layers by means ofhigh-rate cathode sputtering" published by Leybold-Heraeus GmbH. (FRG).In this technique a magnetic field of special distribution is applied tothe target, whereby the electrons are concentrated in front of thetarget and high particle densities are obtained that lead to a reductionin the discharge voltage and lead to higher sputtering rate. Forproducing very hard layers the use of reactive cathode sputtering ispreferable. This technique is used when oxides, nitrides or carbides ofa metallic basic material should be deposited. The basic material, e.g.titanium is used as target. The atmosphere in the discharge chamber is amixture of an inert gate like argon and a reactive gas, e.g. nitrogen.During the sputtering process the reactive gas reacts with the target,and is either resputtered from this, or becomes integrated in thesputtered layer during the condensation of the metal atoms. The hardnessof the layer depends largely on the partial pressure of the reactivegas, and by means of an appropriate process control optimum hardness canbe achieved.

In making the coating 18, the sheets 12 are fed in the discharge chamberand are used as substrates. The coating process is facilitated if a pairof high-rate cathodes are mounted in an opposite position and thesubstrates are placed in the middle zone between the two cathodes. Apreferably equipment for making such coatings is the Modular In-LineSputtering Z 600 of the Leybold-Hereaus GmbH.

A preferable coating is titanium nitride which can have a hardness HV₁₀about 3000 kp/mm² if the partial pressure of the nitrogen is about 5 to10·10⁻⁴ mbar. Similarly hard layers can be obtained by using other kindsof films, such as chromium nitride, silicon carbide, tantalum nitride,tungsten nitride and other hard compounds. The hardness and oxidationbehaviour of such compounds is analyzed in the paper of W. D. Muntz andJ. Gobel "Oxidation behaviour of high rate sputtered TiN, TiC, TiCN, CrNand WN films" published during the 11th ICMC conference in San Diego(Calif.) 1984. There are also a number of publications which deal withthe deposition of hard films on metal substrates, therefore theinvention cannot be limited to any particular compound.

It should be mentioned that the high-rate reactive cathode sputteringtechnique provides for an extremely good cohesion between the substrateand the coating, and this cohesion can surely endure the force andtemperature conditions that prevail in the engagement zone 17 betweenthe tape and the head.

Reference will be made now to FIG. 4 which shows the enlarged sectionalview along line IV--IV of FIG. 3. The scale is distorted for the sake ofbetter illustration. The tape 16 moves normal to the plane of thedrawing and presses the head with pressure P. The support surface of thehead consists of a laminar sandwich-like structure of the iron sheets 12and the coatings 18 thereon. The thickness of the sheets is betweenabout 0.1-0.15 mm, while that of the coating is in the micron range,preferably at least 1 micron. The Vickers hardness HV₁₀ of the structuremeasured along the tape width w can be seen in FIG. 5 in kp/mm² units.The resulting hardness versus width curve is a comb-like formation withabout 3000 kp/mm² peak and 150 kp/mm² basic hardness values. It has beenexperienced that due to the rigidity of the tape material in the shortdistance between adjacent spaced hard coatings, the resulting hardnessof the structure is defined by the coating material and the soft ironsheets 12 have practically no functional role in determining the surfaceabrasion. The significant increases in hardness is experienced even ifthe thickness of the coating was a fragment of a micron, for safetyreasons, however, it is preferable if the coating 18 is at least about0.5 to 1 micron, preferably two microns thick. There is no furtherhardening effect if the coating thickness is increased beyond thisvalue, however, one can produce thicker coatings, too. The soft-ironsheets 12 provide for a good support for the thin coatings, thereforethis latter is definitely fixed between the sheets. Since the sheets 12are made of a heat-conducting metal, the heat generated by the frictionbetween the tape and the head will be transported away, thereforeremarkable local temperature gradients cannot occur. If the headstructure becomes abraded the sandwich structure remains unchanged andthe full depth of the head in the region of the air gap 13 can beutilized. These factors explain why the expected life-time of the headaccording to the invention has increased substantially compared to thatof conventional heads of laminated iron sheets.

FIG. 6 shows the top view of the engagement area viewed through the tapein the case of a double track head. In the engagement zone 17 betweenthe head and the tape there are two sandwich structures 19 and 20,respectively and each of them consists of a plurality of sheets 12 andassociated coatings 18. The tape 16 is wider than the tracks and amagnetic shielding 21 is arranged between the tracks. It is preferableif the shielding 21 is also made of the high permeability iron sheet andit is covered by a coating either on one face or on both. By providing ahard coating on the shield, the hazard of a magnetic short-circuitbetween the tracks is eliminated, since the coating is made of anon-magnetic material. In addition to this advantage, the coating on theshield 21 can provide a further support for the tape. FIG. 6 showsfurther two optional coated sheets 22, 23 on both sides of thestructures 19 and 20, which can be located in the support material ofthe head made generally of copper. The use of coated sheets 22 and 23can provide further supports for the tape 16.

While the coating 18 was mentioned as a layer deposited on the surfaceof the pole sheets, it can well be understood, that separate foils, e.g.tungsten foils or foils of any other hard material can be used insteadof the deposited coating. With present technologies, however, thedeposited coating seems to be far more favourable than separate foils.

The sheets can be coated on both sides or on one side only, or one mightuse coated and uncoated sheets alternatingly. The important thing is toarrange the hard edges with such spacings that the resulting surfacehardness of the frontal zone be defined decisively by them. It is oftenpreferable if the full surfaces of the sheets 12 are coated.

The plane of the sheets 12 and of the coatings 18 need not be normal tothe direction of the tape movement. A tilting on either directions or atilting relative to the plane of the tape is possible. There are severalmagnetic head arrangements, in which tilted heads are used for obtainingincreased channel separation. The presence of the coating 18 dose notlimit the conventional possibilities for arranging the head relative tothe tape.

FIG. 7 shows a simplified perspective view of a multi-channel head witha number of pole structures each comprising sheets coated with a hardfilm. The magnetic shieldings between the channels comprise also hardcoatings. The hardness versus width curve is shown in FIG. 8, in whichthe dashed lines correspond to the shield plates between the channels.

According to the invention the hardness of softiron heads has beenincreased, whereby the lifetime has been increased by a factor of 4 to 8and such heads can as well be used with chromium dioxide and metal tapesas with ferrite and glass ceramic heads. The good magnetic properties ofpermalloy-cored heads remain unchanged, sine the small amount ofnon-magnetic coatings cannot decrease the active iron mass to anoticable extent. The insulating properties of the coating can decreasethe eddycurrent losses, since any current flow between the sheet issafely blocked. A further advantage lies in the preservation of the highproduction rate of conventional sheet-cored heads for manufacturing highquality, long life magnetic heads.

We claim:
 1. A method for producing magnetic recording and/or playbackheads with improved abrasion resistance for use with a magnetic recordmedium having a magnetizable surface for retaining information in theform of a pattern of magnetization, comprising the steps of:(a)providing pole sheets of predetermined shape made of a material withhigh magnetic permeability, each sheet having a frontal zone for slidingengagement with said recording medium and a lateral surfacesubstantially normal to said frontal zone for defining a side of a polegap; (b) providing a hard layer on at least one surface of said sheetsextends to said frontal zone by means of high rate reactive cathodesputtering, said layer having a substantially uniform thickness in therange of 0.5 to 2.0 micrometer, having a Vickers hardness correspondingto the hardness exhibited by titanium nitride, chromium nitride, siliconcarbide, tantalum nitride or tungsten nitride and being of anelectrically non-conductive and non-magnetizable material; (c)assembling said sheets together to form a pair of half cores in such away that in each of said half cores said sheets are stacked on eachother and at least one of said layers 12 arranged between each pair ofsheets in each half core; and (d) mounting said half cores together toform said head.
 2. The method as claimed in claim 1, wherein said layercovers a whole face of each of said sheets.
 3. The method as claimed inclaim 1, wherein said layer is comprised of titanium nitride.
 4. Themethod as claimed in claim 1, wherein said layer covers said lateralsurface of each of said sheets.
 5. The method as claimed in claim 1,wherein said layer has a substantially uniform thickness in the range of0.5 to 1.0 micrometer.
 6. The method as claimed in claim 1, wherein saidlayer has a substantially uniform thickness of at least 1.0 micrometer.7. The method as claimed in claim 1, wherein said pole sheets have athickness in the range of 0.1 to 0.15 mm.
 8. The method as claimed inclaim 1, wherein said layer has a Vickers hardness of about 3000 kg/mm².9. The method as claimed in claim 1, wherein said layer has a Vickershardness as high as 3000 kp/mm².